Therefore, unlike what happens with letter-spacing, 1.2em is not transformed to an absolute length.

Then, you can tell all elements to use --spacing as the value of letter-spacing. So 1.2em will be resolved locally with respect of the font size of each element.

Unlike * { letter-spacing: 1.2em; }, this approach sets --spacing: 1.2em only once, in the body, and lets it propagate by inheritance. Therefore, if you want to change that value in a subtree, you only need to override --spacing in the root. You don't have to select all the subtree.